The invention concerns a method in a paper machine for transfer of the paper web that was formed in the former section to the press section. In the method, a pick-up point is formed in connection with the wet wire of the former section, the paper web being transferred in the area of the pick-up point, using a difference in pressure, onto a pick-up fabric or an equivalent transfer fabric. Before the pick-up point, edge strips are separated from one or both edges of the web by means of cutting grooves, the web between the edge strips or the web being separated from the wet wire at the pick-up point and being passed on support of the pick-up fabric or equivalent to the press section in the paper machine.
Further, the invention concerns a device in a paper machine for transfer of the paper web that was formed in the former section to the press section. In the paper machine, a pick-up point for the paper web is formed in connection with the wet wire of the former section, at which pickup up point the paper web is transferred, while making use of a difference in pressure, onto a pick-up fabric or an equivalent transfer fabric. Before the pick-up point, cutter devices are arranged by whose means edge strips are separated from one or both edges of the web. The web between the edge strips is separated from the wet wire at the pick-up point and is passed on the pick-up fabric or equivalent to the press section in the paper machine.
Increased running speeds of paper and board machines cause new problems to be solved, which problems are mostly related to the runnability of the machine. At present, speeds of up to about 1600 to 1700 meters per minute are employed in paper machines. At these speeds, so-called closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced center roll, as a rule, still operate satisfactorily. As examples of such press sections should be mentioned the current assignee""s Sym-Press II(trademark) and Sym-Press O(trademark) press sections.
In recent years, as running speeds of printing-paper machines, running speeds as high as about 40 meters per second=2400 meters per minute have been contemplated. Achievement of speeds as high as this, in particular in wide machines, provides ever more difficult problems to be solved, some of the most important of which are runnability of the machine and adequate water drain capacity at a high web speed. In a corresponding way, in board machines (basis weight of web  greater than 100 grams per sq. meter), attempts are made to raise the present-day web speeds (8 to 15 meters per second) to a level of 15 to 25 meters per second.
When running speeds of paper machines are increased, the problems of runnability of a paper machine are also encountered with increased emphasis, because a web with high water content and low strength especially does not endure the dynamic forces arising from high web speeds and changes in the running direction of the web, but web breaks and other interference in the operation arise, which will be described in more detail later and which produce standstills or faults of quality in the paper. In a modern printing-paper machine, the cost of break standstill time is currently of an order of 50,000 FIM per hour.
In prior-art press sections, the web is, as a rule, passed from the forming wire into the first press nip on a pick-up felt, which also operates as a press fabric that receives water in the first press nip, which nip is either a roll nip or an extended nip. In the first press nip, it is necessary to use a relatively high compression pressure and to deal with large quantities of water, and it is one drawback arising from this that the outer face of the press felt tends to be contaminated and its porous fabric structure to be partly blocked. Attempts are made to prevent this by means of efficient felt conditioning devices, which are, however, components that are quite expensive, spacious, and consume an abundance of energy. A further drawback of the press felts used in prior-art press sections is their effect of rewetting the web.
In a way known from the prior art, in the vicinity of the wet wire, before the pick-up point, devices for cutting of edge strips off the web, i.e. so-called edge spray cutters, are fitted, by whose means, by means of water jets applied to the web placed on the wet wire, narrow edge strips are cut off from both edges of the web so that a web of uniform width with whole and good edges is obtained to be passed to the press section. As is known from the prior art, the cutters of edge strips can be provided with position regulation devices, by whose means the web cutting width can be adjusted and regulated within certain limits.
By means of the present invention, attempts are made to eliminate problems related to the cutting of the edges of the paper web which have been encountered in the current assignee""s test machine when running at a high speed and in particular with webs of relatively low basis weight. When the running speed of the test machine was higher than about 1500 meters per minute, the edge strips that were cut off from the paper web in the wire part before the pick-up roll tended to follow the pick-up felt after the pick-up roll, instead of following the wet wire. This took place in particular when the permeability of the pick-up felt was low. The permeability of the pick-up felt can be low also when new, because of its structure, and in particular when the felt is compacted and contaminated as a result of prolonged service life, as was described above. If the edge strips of the paper web enter fully or partially into the press nips, they cause interference with the runnability and produce fiber strings that damage the press felts and the extended-nip belts.
The above problems related to the cutting of the edges of a paper web and the other difficulties are emphasized further when the web speed approaches 2000 meters per minute and exceeds the speed. It is one of the problems of runnability that, owing to these problems, in the press section it is not possible to employ steam boxes in order to enhance the dewatering. It has not been possible to a major extent to reduce these problems by means of regulation of the cutting width of the water jet devices that cut the edges of the paper web, i.e. of the edge spray cutters, or by means of regulation of the suction width in a pick-up roll.
It is probably one of the reasons for the problems related to the cutting of the edges of the paper web that replacement air flows into the opening nip between the forming wire and the pickup felt more readily through the forming wire than through the pick-up felt. Since the width of the strip torn off from the edge is approximately equal to the width of the end seals of the suction zone in the pick-up roll, these seals may prevent the flow of replacement air through the holes in the mantle of the pick-up suction roll at the location of the seals. This is also indicated by the fact that it can also be frequently seen in production machines that the edge strips are folded double across a width of about 30 mm after the pick-up point before the edge strip knock-off jets.
By means of the present invention, attempts are made to provide novel solutions for the problems dealt with above, in particular when the web speed is higher than 1500 meters per minute and, as a rule, in a range 1500 to 2500 meters per minute.
It is a particular object of the invention to provide a method and a device of the sort concerned by whose means it can be ensured that either the edge strips run after the pick-up point along with the wet wire or the edge strips are separated from the pick-up felt so that the edge strips can be passed in a controlled way, by the effect of centrifugal forces and, if necessary, by means of knock-off water showers, into a wire pit or equivalent placed underneath.
In view of achieving the objectives stated above and those that will come out later, the method in accordance with the invention for transferring a paper web from a wet wire in a former section to a pick-up fabric or an equivalent transfer fabric of a press section includes forming a pick-up point on the wet wire and separating an edge strip from at least one edge or both edges of the web prior to the pick-up point by means of at least one cutting device. The web is separated from the wet wire and passed onto the pick-up fabric at the pick-up point. The method further includes applying a pressure difference to the edge strips through the wet wire or the pick-up fabric at or after the pick-up point to cause the edge strips to either follow the wet wire or become separated from the pick-up fabric after the pick-up point and to be placed in a broke system arranged beneath the paper machine.
On the other hand, the paper machine in accordance with the invention in which a paper web is transferred from a wet wire in a former section to a pick-up fabric in a press section in a running direction of the web includes a pick-up point located on the wet wire and at least one cutter device arranged in association with the wet wire before the pick-up point in the running direction of the web for separating an edge strip from at least one edge or both edges of the web. The paper machine also includes means for transferring the web from the wet wire to the pick-up fabric at the pick-up point and means for applying a pressure difference to the edge strips through the wet wire or the pick-up fabric at or after the pick-up point to cause the edge strips to either follow the wet wire or become separated from the pick-up fabric after the pick-up point and to be placed in a broke system arranged beneath the paper machine.
In a device in accordance with a preferred embodiment of the invention, the edge strips of the web are kept in contact with the wet wire by vacuum devices of a width substantially equal to the width of the edge strips. An alternative with a full-width vacuum device is also possible, in which case intensifying suction zones can be fitted in the areas of the edge strips. A full-width alternative of a vacuum device may be provided with instantaneous switching off of the vacuum in the middle area as soon as threading of the web starts. By means of this method and device it is possible to improve the quality of the tip of the web in threading, and also in cutting-off the web, when the vacuum of the vacuum device is switched on at a suitable stage.
In accordance with a second preferred embodiment of the invention, pressure devices of a width substantially equal to the width of the edge strips and placed after the pick-up roll and fitted at the location of the edge strips are used, by means of which pressure devices the edge strips are separated from the pick-up felt and passed, for example, by means of a guide plate or plates down to the broke system.
In accordance with a third preferred embodiment of the invention, pressure zones are used, which are placed outside the vacuum zone of the pick-up roll at both sides of the vacuum zone and which have been fitted at the location of the edge strips and which prevent portions placed outside the paper web proper to be passed to the press section from following the pick-up felt.
In a fourth embodiment of the invention it is possible to arrange a blow device of full width inside the loop of the wet wire after the pick-up point, by means of which blow device the web of full width can be separated from the wet wire when the web is not passed to the press section, e. g., in the event of a disturbance in operation.
The alternative embodiments of the method and the device mentioned above can be used each of them alone or as a suitable combination, always as necessary.
The present pick-up method and devices are, as a rule, applied in paper machines whose speed is higher than xcx9c1500 meters per minute. A pick-up method and a device in accordance with the present invention are particularly useful, and in many cases even indispensable, in paper machines whose speed range is 1700 to 2500 meters per minute, in which speed range the problems that are supposed to be eliminated by means of the present invention occur with more emphasis when the speeds that are used become higher.
By means of the method and the device in accordance with the invention, it is possible to ensure in all different situations of operation, also at very high speeds, that the edge strips do not enter into the press section along with the pick-up felt, but the edge strips either follow the wire directly after the pick-up point and fall down into the broke system placed underneath, or are separated from the pick-up felt at the latest in or after the pick-up zone and are passed, for example, by means of a guide plate, into the broke system. Further, in a preferred embodiment of the invention, it is ensured that the web of full width is separated from the wet wire in a situation of operation in which the web is not passed to the press section, for example, in the event of a disturbance in operation, but the web is passed from the wet wire directly into the broke system.
In the following, the invention will be described in detail with reference to some environments of application and exemplifying embodiments of the present invention illustrated in the figures in the accompanying drawing, the invention being in no way strictly confined to the environments or embodiments.